The goal of corn breeding is to produce a hybrid which has outstanding agronomic features for a number of traits. The most important trait is typically the grain yield expressed in weight or volume of grain produced per unit area of harvest, e.g., tons/hectare. Several secondary traits, many of which indirectly affect yield, are also important. These traits may include resistance to disease and insects, resistance to lodging, tolerance to heat and drought, length of time to maturity, and quality traits. Any new hybrid developed by breeding techniques must have improvement in enough of these traits to provide a grower an advantage over previously released hybrids.
The breeding and development of new commercial hybrids involves the use of 2 essential techniques: self-pollination and cross-pollination. For self-pollination, the pollen from one plant is placed on the silks of the same plant. For cross-pollination, pollen from one plant is placed on the silks of another plant. When self-pollination is practiced over a series of several generations, the resulting plants become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. Plants at this stage in development are called inbred lines and can be maintained by continued self-pollination.
A cross between two homozygous, inbred lines produces a uniform population of plants referred to as F1 hybrids. F1 is defined as the first filial generation resulting from a cross. While these hybrids are genetically uniform, they are typically heterozygous for many gene loci. A cross of 2 plants that are not inbred will also produce F1 plants with many heterozygous loci; however, the hybrid plants resulting from this cross will not be genetically uniform.
During the self pollination process, inbreeding occurs and the vigor of the lines decrease. The vigor, which is manifested in many ways including increased vegetative growth and increased yield, is restored when two unrelated inbred lines are crossed to produce the F1 hybrid seed. It is important that lines used as parents in commercial FI hybrids be inbred (true breeding) and unrelated. The inbred nature ensures the uniformity of the hybrid, which is essential for mechanical harvesting and it allows the reproduction of the same hybrid year after year. The unrelatedness of the inbred lines maximizes the vigor that result in the F1 hybrids. Hybrids between closely related lines are not as vigorous as from unrelated lines.
A single-cross hybrid is produced when two inbred lines are crossed to produce the F1 progeny. A double-cross hybrid is produced from four inbred lines crossed in pairs (A.times.B and C.times.D) and then the two F1 hybrids are crossed again (A.times.B).times.(C.times.D). Double cross hybrids are not as genetically uniform as single-cross hybrids; however, they can be uniform enough to be acceptable for mechanical harvest procedures used today.
The development of commercial corn hybrids requires: 1) the development of homozygous inbred lines, 2) the crossing of these inbred lines to form hybrids, and 3) the evaluation of these hybrids.
The development of inbred lines most commonly utilizes the pedigree method of breeding. The initial step involves the development of a source population. The source population may consist of an F1 hybrid, or it may consist of a group of plants resulting from cross pollination of several lines. Lines to be included in the source population are selected so that desirable traits from different parental lines will be combined together into the daughter population. This daughter population is self-pollinated several generations, with the best plants selected during each generation of inbreeding. As the lines become more inbred, the genes included in the source population will recombine in new combinations giving a series of new inbred lines, some having desirable traits from different parents involved in the source population.
These new inbred lines are then crossed to other, unrelated inbred lines to produce F1 hybrids which are evaluated in multiple locations for commercial potential. The best F1 hybrids are identified and the inbred lines used to make these F1 hybrids are then increased for commercial production.
Backcrossing can also be used to improve an inbred line. Backcrossing transfers a specific desirable trait from one inbred or source to an inbred that lacks a trait(s). This can be accomplished for example by first crossing a superior inbred (A) (recurrent parent) to a donor inbred (non-recurrent), which carries the appropriate gene(s) for the trait in question. The progeny of this cross is then mated back to the superior recurrent parent (A) followed by selection in the resultant progeny for the desired trait to be transferred from the non-recurrent parent. After several backcross generations, the progeny will contain the new trait in the superior background of the recurrent parent.
Other breeding methods for developing inbred lines include recurrent selection and various modifications of the pedigree method.
While the ultimate goal is to identify superior hybrids, this must be achieved by identifying superior inbred lines as parents of the hybrids.